1. Field of the Invention
The present invention pertains to systems for scanning surface patterns on specimens such as semiconductor wafers or the like, and more particularly, it pertains to methods and apparatus for accurately obtaining measurements of the surface profile of such specimens.
2. Description of the Prior Art
In the scanning of semiconductor wafers or the like to detect surface pattern defects, a variety of techniques have been utilized that take advantage of various forms of microscopes, both optical and acoustical, having high degrees of image resolution. In optical imaging systems generally, devices similar to T.V. cameras have been utilized wherein electromagnetic radiation is reflected from a relatively large spot on the wafer and processed through an optical system and photodetector device to provide a multi-intensity image which, either digitally or by analog means, can be recreated on an appropriate output device, such as a CRT.
The scanning of semiconductor wafers typically provides a means whereby certain processing defects can be detected or whereby linewidth measurements can be made so as to determine whether or not the manufacturing process has been performed correctly. Since the tolerance limits for the dimensions which must be detected and measured accurately are in the micron or even submicron range, microscope imaging systems for scanning specimens with a high degree of resolution are generally required. Laser beams can be focused through such optical imaging systems with a very narrow depth of field. Then, by scanning the laser beam along the top surface of the semiconductor wafer, the conductive traces or conductor pattern lines on the wafer can be measured by utilizing special detector devices to denote the edges of such lines.
It has been generally recognized that with wafer imaging and scanning systems of the aforedescribed type the beam focus level can be adjusted as it is scanned across the wafer so as to track the changing surface level thereof by noting when the reflected image moves slightly out of focus and by adjusting the spacing between the wafer and the imaging system (by moving either one relative to the other) so as to continually maintain the reflective surface of the wafer at the proper focus. Prior art patents which describe such imaging systems include U.S. Pat. No. 4,505,585 to Yoshikawa et al and U.S. Defensive Publication T102,104 to Kirk et al.